<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .block {
            width: 100px;
            height: 100px;
            background-color: blue;
            transition: all 1s;
        }

        .block:hover{
            width: 300px;
        }
    </style>
</head>
<body>
    <div class="block">

    </div>
</body>
</html>
